Direct bonding allows substrates to be obtained in which the active layer and the receptor substrate are directly bonded to each other without an interlayer, without a bonding layer such as an insulating layer for example. For example, a direct bonding between an active layer and a substrate having different crystal orientations, or more generally different crystallographic characteristics, can thus be envisioned.
However different crystalline characteristics may give rise to crystal defects at the interface between the active layer and the receptor substrate, such as dislocations likely to spread into one or the other of the layers. Moreover, the surface treatments designed to improve hydrophilic or hydrophobic bonding behavior produce a modification of the interface with, for example, the formation of oxygen precipitates. Indeed, during hydrophilic bonding a very thin oxide film (of the order of several atomic layers, that is of a thickness of 5 to 10 angströms) is formed on the surface of the substrates.
In the case of hydrophobic bonding this film does not exist, but the substrate itself, ever since its manufacture from ingots, contains oxygen atoms within its lattice. The various treatments the wafers undergo before being brought into contact will lead to the formation of precipitates, clusters formed by rearrangement, agglomeration of these various oxygen atoms. These defects are typically small discontinuous polygonal or spherical structures of a size of the order of several nanometers in diameter. Such defects impair bonding behavior and consequently the quality and the performance of components that will subsequently be fabricated on the substrate.
The article by Bourdelle et al. (“Fabrication of Directly Bonded Si Substrates with Hybrid Crystal Orientation for Advanced Bulk CMOS Technology”, ECS Transactions, 3 (4) 409-415 (2006)) thus proposes a method for fabricating DSB substrate according to SMARTCUT® layer transfer technology via hydrophilic bonding between a donor substrate made of silicon in (110) crystal orientation and a receptor substrate made of (100) silicon. A thin layer of sacrificial oxide is deposited on the donor substrate before the following implantation step of bringing the two substrates into intimate contact. After steps for adequate finishing, a heat treatment is carried out in order to dissolve the residual oxide and to form a conductive bonding interface. The layer of sacrificial oxide serves as a shield to protect the surface of the donor substrate from contamination by particles or by metals, which allows an interface without defects and a thin layer from the donor substrate having a very high crystallographic quality to be obtained.
Furthermore, methods are known of fabricating hybrid substrates, that is substrates with an active layer having regions of different crystal orientation, using the amorphization technique. In this respect, the article by Saenger et al. (“Amorphization/templated recrystallization method for changing the orientation of single-crystal silicon: An alternative approach to hybrid orientation substrates”, App. Phys. Lett., 87, 221911, 2005) may be referred to, which describes, for a substrate produced with an active layer of silicon in a first orientation bonded onto a support substrate also made of silicon but having a second orientation, the amorphization of certain areas of the active layer, then their recrystallization according to the crystal orientation of the support substrate. However, this method consists in amorphizing the active layer across its whole thickness, through to a depth situated beyond the interface. In addition, this method leads to a recrystallization of the active layer according to the crystal orientation of the support substrate, so that in the areas having undergone this treatment the interface can no longer be distinguished.
One of the aims of the invention is therefore to propose a method for treating a DSB substrate formed by bonding an active layer from a donor substrate onto a receptor substrate, improving the interface quality or shifting the defects present at the interface inside the structure, while conserving the crystalline characteristics of the active layer and of the receptor substrate.